1. Field
This invention pertains to the absolute calibration of receiving systems and, in particular, to the calibration of wideband systems, such as panoramic radio receivers, scanning receivers and spectrum analyzers.
2. Prior Art
Frequency scanning superheterodyne receivers are well known. With the development of modern microwave devices, the frequency range of such receivers has been extended to frequencies well beyond 20 GHz. The ability of these receivers to continuously sweep over wide bandwidths has also been extended, as illustrated by the multioctave tuning range of the receiving system described in U.S. Pat. No. 3,701,949.
The increase in tuning range of such receivers has not been achieved without drawbacks. One of the major problems of wideband receivers is the variation in gain across the RF frequency range. These variations in gain are caused by changes in the transfer characteristics of the RF components and also by mistracking between the preselector and local oscillator. The typical variations in gain of such receivers over a band of 2 to 12 GHz can be as high as .+-. 6 dB.
Conventional amplitude calibration procedures may be used to correct for these gain variations over limited frequency ranges; however, such procedures usually require, as a minimum, a signal generator and power meter capable of covering the narrow frequency range of interest.
In calibrating a receiver with this equipment, the power supplied by the signal generator to the receiver is monitored by the power meter and the output level of the receiver is compared to the readings of the power meter. Over relatively narrow frequency ranges, the data provided by the power meter and output level indicator may be manually recorded and a correction chart made for correcting the data produced by the receiver.
Over wide frequency ranges, the cost and labor involved in carrying out manual calibration and correction procedures are prohibitive, making an automatic system a practical necessity. A wideband automatic system is capable of eliminating the high labor cost associated with manual calibration systems, but a conventional automatic system inherently require a significant capital investment in a wideband signal generator and power meter as well as automatic recording equipment.